1. Beginning Astroimaging
Session 3
Written by Geoff Smith

2. Session Summary
Objective: Improve image quality by combining multiple pictures of a target Motivation: • Understanding image stacking • See how it improves image quality Equipment: • Stacking software, e.g. Deepsky Stacker, CCDStack, PixInsight Content: • Aligning the pictures and stacking, data rejection • Short overview of image processing software  • Image calibration briefly mentioned

3. What is stacking and why should I care?
Digitally combining several exposures is, in an ideal world, equivalent to taking one exposure of the same total length.
Example: 6 exposures of 10 minutes is very nearly equivalent to a single 1 hour exposure
Stacking gives the advantage of a long exposure time without the disadvantages (next slide).

4. Why not take one long exposure?
Taking a single one hour exposure assumes that everything will work perfectly for an hour. Some things that can cause problems with this expectation are:
Tracking errors
Satellite or aircraft trails
Wind gusts
The old kick-the-tripod-in-the-dark trick
Poor polar alignment
It is much less painful to throw out one ruined 10 minute exposure than to throw out a 1 hour exposure.

5. What we are leaving out for now.
At this stage we are trying to keep things simple while achieving a reasonable result. Later we shall refine our methods by considering the following topics:
Flat Frames (Flats): These are used to counter instrumental defects such as vignetting, dirt on the camera sensor and pixel to pixel variation in camera sensitivity.
Dark Frames (Darks): Camera sensors will generate a signal even when an exposure is made in the absence of light. This is the so-called Dark Current which can be removed by use of dark frames.
Bias Frames (Biases): These are exposures of the shortest possible duration taken in the absence of light. They are a necessary adjunct to flat frames and are sometimes used with dark frames.
The use of flats, biases and darks to improve our images is known as Image Calibration and is very important in getting the most out of our data. We will cover this topic in Session 7

6. Mechanics of Stacking
The first step is to align all your images. This is known as Image Registration, Image Alignment, Star Alignment, etc, etc. Any astrophotography software will enable you to do this.
Once aligned we can digitally combine the images. This is (Image) Stacking or (Image) Integration.

7. Stacking Explained Take the average Aligned Subframes
of all these pixels
Aligned Subframes
To get this pixel
Stacked frame
This process is done for all pixels in the subframes

8. Sum or Average—What’s the difference?
The first thought that many of us have is:
Why take the average, won’t the sum give a better signal?
The answer is “NO” because the sum and the average contain exactly the same information.
If I know the sum of n images, I can retrieve the average by dividing all the pixel brightnesses by n.
If I know the average of n images I can retrieve the sum by multiplying all the pixel brightnesses by n.
Information is not affected by constant multiplication.
The main reason we don’t take the sum is that bright objects may overflow the possible range of brightnesses expressible in the image

9. Astronomical Image Processing Tools
These are many dedicated astronomical image processing tools that will stack images:
Astroart (€135)
CCDStack ($ USD)
Deep Sky Stacker (Free) Limited to calibration and stacking—recommended while you are still dipping your feet in the water
MaximDL ($199.00–$ USD) Does much more than processing—telescope control, image acquisition, guiding, observatory control, image processing.
PixInsight (€230) Most commonly used software dedicated to astrophotography processing.
Star Tools ($65 AUD)
Note that Photoshop has not been included on the list. Although capable of superb results, it was never designed to process astrophotographs.

10. All software comes with a learning curve, often quite steep
However, registration and stacking is straightforward
in most software packages:
Load files into the appropriate window
Hit “OK” or something equivalent.
We will illustrate the process with screen shots from
Deep Sky Stacker and PixInsight

11. Deep Sky Stacker
Click here

12. Click

13. If you click on
“Register checked pictures”
you will be given the option
to stack as well.
The stacked file will be auto-saved
in the same directory
Select all images
or else do it manually
Use these to register
and/or stack the images
Images to be registered
and stacked

14. Results of stacking
Single image
Stack of 9 images

15. PixInsight Reference file—usually the best one
Lots of bells and whistles for
advanced users, but the defaults
usually work pretty well
Files to be registered
Registered files will be
written in this directory
Click here to
start the process

16. The stacked file will be written to
screen
Aligned images
Data rejection method
(Explained in the next slide)
Click to begin

17. Data Rejection
The behaviour of light causes a variation from subframe to subframe. Think of photons falling like raindrops onto a piece of paper. Every time you put the paper in the rain for a short time you get a different distribution of drops. Averaging subframes smooths out this variation.
Some pixels may differ greatly from the average—for example a satellite trail. By using various statistical methods, these deviant pixels can be excluded from the stack. This is known as Data Rejection and can be implemented with all stacking software.
This means that you can still use an image with a satellite trail or some other minor defect without the problem showing in the stacked image

18. Further information
Remember that these sessions are a guide to point you in the right direction. You now know about stacking and why it is important. It’s up to you to explore this further.
YouTube has a wealth of information. Do a search on “Stacking astroimages” or “Stacking astroimages in <software name>”. You will get many hits.

19. Next Session
Objective: understand the various cameras you will need for astroimaging Motivation: what you need to know about cameras and how to make a choice Covers: • imaging camera • guide camera • why you want/don’t want a dedicated astroimaging camera • matching camera and telescope • field of view considerations • focal length considerations • colour or mono • software for image acquisition e.g. Sequence Generator Pro, MaximDL, Astroart.